Coccidioidomycosis, otherwise known as the San Joaquin Valley Fever, is a fungal respiratory disease of humans and wild and domestic animals, which is endemic to southwestern United States, northern Mexico, and numerous semiarid areas of Central and South America (Pappagianis, D. Epidemiology of Coccidioidomycosis. Current Topics in Medical Mycology. 1988. 2:199-23). Infection occurs by inhalation of airborne spores (arthroconidia) produced by the saprobic phase of Coccidioides spp., which grows in alkaline desert soil, followed by morphogenic conversion of the fungus to the virulent, parasitic phase in the host mammal.
Coccidioides immitis was the first described species, and is now becoming known as the Californian species. The C. posadasii species was recently defined, and was previously recognized as the non-Californian population of C. immitis (Fisher, M. C., Koenig, G. L., White, T. J., Taylor, J. W. Molecular and phenotypic description of Coccidioides posadasii sp. nov., previously recognized as the non-California population of Coccidioides immitis. Mycologia 2002. 94(1):73-84, 2002). The differences in the two species are slight.
It is estimated that 100,000 new cases of this disease occur annually within the rapidly growing population of people who live in regions of the United States between southwest Texas and southern California, where the disease is endemic (Galgiani, J. N. Coccidioidomycosis: A regional disease of national importance; rethinking our approaches to its control. Annals of Internal Medicine. 1999. 130:293-300). Although the majority of immunocompetent individuals are able to resolve their Coccidioides spp. infection spontaneously, the level of morbidity associated even with the primary form of this respiratory mycosis warrants consideration of a vaccine against the disease. Immunocompromised patients, including those infected with human immunodeficiency virus, are at high risk to contract disseminated coccidioidomycosis (Ampel, N. M., C. L. Dols, and J. N. Galgiani. Results of a prospective study in a coccidioidal endemic area. American Journal of Medicine. 1993. 94:235-240). It is also apparent from results of several clinical studies that African-Americans and Asians are genetically predisposed to development of the potentially fatal, disseminated form of the respiratory disease (Galgiani, J. N. 1993. Coccidioidomycosis. Western Journal of Medicine 159:153-171).
Chitin, a linear polymer of N-acetylglucosamine, is one of the major structural components of the fungal cell wall, and is required for cell shape and morphogenesis. The enzyme chitinase hydrolyzes the 1,4-beta-linkages of N-acetyl-D-glucosamine polymers of chitin, and has been shown to play an important role in molding and shaping the cell wall of many fungi during growth and reproduction (Kuranda, M. J. & Robbins, P. W. 1991. Chitinase is required for cell separation during growth of Saccharomyces cerevisiae. Journal of Biological Chemistry 266, 19758-19767; Takaya, N., Yamazaki, D., Horiuchi, H., Ohta, A. & Takagi, M. 1998. Cloning and characterization of a chitinase-encoding gene (chiA) from Aspergillus nidulans, disruption of which decreases germination frequency and hyphal growth. Bioscience, Biotechnology, and Biochemistry 62, 60-65). In Coccidioides spp., the literature has reported two chitinase (CTS) genes (Pishko, E J; Kirkland, T N; Cole, G T 1995. Isolation and characterization of two chitinase-encoding genes (cts1, cts2) from the fungus Coccidioides immitis. Gene 167:173-7; Cole G T, Hung C Y. 2001. The parasitic cell wall of Coccidioides immitis. Medical Mycology 39 Supplement 1:31-40), and an additional five have been identified by us. These chitinases were grouped into two classes, bacterial-like and fungal-like, according to phylogenic analysis with other reported chitinases. Although evidence suggests that chitinases associate with the segmentation apparatus of parasitic phase Coccidioides spp., the specific role or function of the individual chitinases is not understood. Indeed, deletion of the CTS1 gene (a bacterial-like chitinase) from a strain of Coccidioides posadasii led to no discernible effects on reproductive endosporulation or virulence (Reichard, U., C. -Y. Hung, P. W. Thomas, and G. T. Cole. 2000. Disruption of the gene which encodes a serodiagnostic antigen and chitinase of the human fungal pathogen Coccidioides immitis. Infection and Immunity 68:5830-5838). However, fungal-like chitinases (CTS2, CTS3, and CTS4) of Coccidioides are more likely involved in morphogenesis based on reported data as referenced above (Kuranda & Robbins. 1991, Takaya et. al. 1998).
The rationale for commitment of research efforts to develop a Coccidioides spp. vaccine is based on clinical evidence that individuals who recover from the respiratory coccidioidomycosis disease retain effective long-term cellular immunity against future infections by the pathogen (Smith, C. E. 1940. American Journal of Public Health 30:600-611). In addition, early preclinical studies demonstrated that a formalin-killed whole-cell (spherule) vaccine prevented deaths in mice after infection with even very large numbers of coccidioidal spores (Levine et al. 1961. Journal of Immunology 87:218-227). However, when a similar vaccine preparation was evaluated in a human trial, there was substantial local inflammation, pain, and induration at the injection site, rendering the vaccine unacceptable (Pappagianis et al. Evaluation of the protective efficacy of the killed Coccidioides immitis spherule vaccine in humans. American Review of Respiratory Diseases. 1993.148:656-660). Further, there was no difference in the number of cases of coccidioidomycosis or the severity of the disease in the formalin-killed spherule vaccinated group compared to the placebo group. Therefore, the original human vaccine trial was not successful.
Other attempts to identify a suitable vaccine have focused on the creation of attenuated, live strains of C. immitis for the induction of an immune response. In two such attempts, investigators induced auxotrophic mutations in strains of C. immitis via X-ray irradiation (Foley, J. M, Berman, R. J., and Smith, C. E. X-ray irradiation of Coccidioides immitis arthrospores: survival curves and avirulent mutants isolated. Journal of Bacteriology. 1960. 79:480) or UV-irradiation and chemical mutagenesis (Walch, H. A. and Walch. R. K. Studies with induced mutants of Coccidioides immitis. In L. Ajello (ed.) Proceedings of the Second Symposium on Coccidioidomycosis. University of Arizona Press. 1960. p 339), and then utilized the attenuated strains as vaccines prior to challenging the animals with wild-type C. immitis. However, these and subsequent reports utilizing these strains (Pappagianis, D., Levine, H. B., Smith, C. E., Berman, R. J. and Kobayashi, G. S. Immunization of mice with viable Coccidioides immitis. Journal of Immunology. 1961. 86:28; Walch, H. A. and Walch. R. K. Immunization of mice with induced mutants of Coccidioides immitis. I. Characterization of mutants and preliminary studies of their use as viable vaccines. Sabouraudia. 1971. 9:173) demonstrated that although varying degrees of immunization were attained with these strains, the attenuated strains nevertheless were capable of converting to the parasitic phase and resulted in localized or disseminated lesions in the experimental animals. In one instance, the attenuated strain regained its virulence through the loss of the auxotrophic state, causing disease in vaccinated animals. Given the evidence of localized or disseminated disease, the investigators found the attenuated strains to be inappropriate as vaccines.
Therefore, there is a long felt need for more effective, safe and usable compositions for inducing an immune response to prevent, treat, or ameliorate infection of Coccidioides spp. and disease states associated with the infection.